Human periventricular leukomalacia (PVL) is the major form of cerebral white matter injury underlying cerebral palsy and a leading cause of chronic neurological deficits in survivors of premature birth. With advances in neonatal intensive care, diffuse white matter injury has emerged as the principle form of PVL, characterized by prominent activation of microglia and astrocytes, selective loss of premyelinating oligodendrocytes (preOLs) and disturbances in myelination. Inflammation in the immature brain is associated with hypoxia/ischemia and maternal/fetal infection. However, the mechanism by which it contributes to preOL destruction remains uncertain. We demonstrated previously that activated microglia directly kill preOLs through production of peroxynitrite and that astrocytes switch this preOL death mechanism to one dependent upon tumor necrosis factor (TNF). We subsequently found that reactive astrocytes in human PVL accumulate significant amounts of ceramide, a signaling sphingolipid with critical roles in cell death and inflammation, and that ceramide acts synergistically with TNF causing apoptotic death of preOLs through an astrocyte dependent mechanism. Moreover, astrocytes markedly promote TNF production by activated microglia. As TNF is generated in PVL, our results suggest that astrocytes play an important role in amplifying cytokine toxicity to preOLs, thereby augmenting diffuse white matter damage. Thus, in the previous grant cycle, we identified a novel, pathogenic process in which activated astrocytes and microglia act co-operatively in regulating preOL demise. In the renewal, we hypothesize that astrocytes amplify microglial proinflammatory responses through induction of galectin-9 and that disruption of ceramide metabolism in astrocytes predisposes preOLs to inflammatory destruction. Our specific aims are (1) to determine the function of galectin-9 in regulating microglia immune responses; (2) to determine the mechanism by which altered ceramide metabolism in astrocytes potentiates preOL injury; and (3) to investigate whether modulating sphingosine-1-phosphate signaling pathway confers protection in a rat model of PVL. We will use galectin-9 deficient mice to establish the function of galectin-9 in regulating CNS inflammatory responses. We will also employ liquid chromatography tandem mass spectrometry to monitor changes of bioactive sphingolipids under various experimental paradigms and identify specific sphingolipid pathways involved in preOL injury and protection. This project is likely to generate novel insights into the mechanism of preOL destruction, and will provide the framework for targeting specific sphingolipid signaling pathways for therapeutic interventions of white matter injury.